major reference

Classical Sanskrit represents a development of one or more such early Old Indo-Aryan dialects. At this state, the archaisms noted above have been eliminated. For all this simplification, Classical Sanskrit is considerably more complex than Middle Indo-Aryan. In addition to the vowels
a, i, and
u (in both long and short varieties), it has
ṛ and...

use in Sanskrit writings

What is generally called Classical Sanskrit—but is actually a language close to late Vedic as then used in the northwest of the subcontinent—was elegantly described in one of the finest grammars ever produced, the
Aṣṭādhyāyī (“Eight Chapters”) composed by Pāṇini (
c. 6th–5th century
bce). The...